How an Atkinson Cycle Engine Works

The Atkinson Cycle and Hybrid Cars

So, you've got an engine that's really efficient, but it lacks in power, especially of the torque variety, the kind of power that the fire-breathing drag car has in spades. But, if you're a hybrid powertrain engineer, you also have an electric motor that has all torque all the time, from 0 rpm. The problem with the electric motor is that it doesn't sustain a high speed very well, not as well as a gasoline engine does, with its higher horsepower. What to do, hybrid powertrain engineer?

Well, if you're Gilbert Portalatin, who happens to be a hybrid powertrain engineer with Ford, or any other engineer at almost any other car company building full hybrids, you bring these two systems together like chocolate and peanut butter. At low speeds, the electric motors kick in with their torque and move the car forward. Unless you're one of those super careful hypermilers who press the accelerator as gently as if a kitten were hiding underneath it, the gasoline engine will come online pretty quickly, though the electric motor is doing quite a bit of work. At about 40 mph or so, the Atkinson cycle engine will take over almost completely, with a bit of assist from the electric motor.

As long as you've got this kind of combo, you can engineer the Atkinson cycle engine to mesh precisely with the electric motor for optimal efficiency. If you insist on taking on the fire-breather in the next lane, you won't be left completely in the dust. "Smash the pedal, and you'll get what you're asking for -- all of both powerplants," said Lee at Toyota.

This load leveling is why a full hybrid like the Toyota Prius or Ford Escape get better mileage around town than they do on the highway -- exactly the opposite of, like, every other vehicle on the road. The non-fire-breathers among us drive pretty slowly around town. We start and stop a lot, and we don't get up to 75 mph, so the electric motor takes a lot of the burden. On the highway, though, the gasoline engine is pretty much working alone.

Hardly anyone in 1887 could have predicted the happy peanut butter-and-chocolate marriage between Atkinson's engine and electric motors -- cars didn't even have permanent roofs then.